When forced with increasing greenhouse gases, global climate models project a delay in the phase and a reduction in the amplitude of the seasonal cycle of surface temperature, expressed as later minimum and maximum annual temperatures and greater warming in winter than summer. All 24 CMIP3 models agree on these changes and, over the 21st century, average a phase delay of 5 days and an amplitude decrease of 5% for the global mean ocean surface temperature. Most of the global mean changes come from the high latitudes, especially over ocean. We provide evidence that the changes are mainly driven by sea ice loss: as sea ice melts during the 21st century, the previously unexposed open ocean increases the effective heat capacity of the surface layer, slowing and damping the temperature response. From the tropics to the midlatitudes, changes in phase and amplitude are smaller and less spatially uniform than near the poles, but they are still robust. These regions experience a small phase delay, but an amplitude increase of the surface temperature cycle, a combination that is inconsistent with changes to the eective heat capacity of the system. We propose that changes in this region are controlled by changes in surface heat fluxes.